Interstand tension control for tandem cold rolling mills



A. `v. slLvA April 21, 1970 INTERSTAND TENSION CONTROL FOR TANDEM GOLDROLLING MILLS Filed Feb. 2o, 1968 3 Sheets-Sheet 1,

Ommam AprilZl, 1970 A. v. slLvA INTERTAND TENsLou-CONTROL Fon TANDEMcoLD ROLLING MILLS Fled Feb. 20, 1968 i f3 Sheets--Sheei'l 2 ICef,

sTANo` sTAN STA D v:5 4 D 5N FROM 5TH STAND TACHOMETER F|G.2.

wlmesses: |NvENToR @wwwa e Anto io V. Slvvo ATTORNEY April 2l, 1970INTERSTANOTENSION CONTROL FOR TANDEM'COLD ROLLING MILLS Filed Feb. 20,1968 5 Sheets-Sheet 5 United States Patent O 3,507,134 INTERSTANDTENSION CONTROL FOR TAN- DEM COLD ROLLING MILLS Antonio Vicente Silva,Sloan, N.Y., assignor to Westinghouse Electric Corporation, Pittsburgh,Pa., a corporation of Pennsylvania Filed Feb. 20, 1968, Ser. No. 707,393Int. Cl. B21b 37 00 U.S. Cl. 72-8 7 Claims ABSTRACT OF THE DISCLOSUREThe method and apparatus for the control regulation of the respectiveinterstand tensions and thereby the rolled strip thickness in a tandemrolling mill by a predetermined combination of stand screwdownadjustment and stand speed control. In the operation of a tendem rollingmill, adjacent stands are coupled by the strip tension; and during thedesired operation of the rolling mill, whereby the nal thickness of thestrip delivered from the mill is held substantially constant, therespective interstand tensions should be controlled to be substantiallyconstant such that the samevolume of material should pass through eachstand per given unit of time. When there is obtained this continuitycondition on the volumetric ow of material through the rolling mill,with the interstand tensions held substantially constant, the followingrelationship exists between the thickness h of strip delivered from eachstand and the speed v of the strip leaving each stand,

BACKGROUND OF THE INVENTION For the steady-state operation of a tandemmill, the volume of metal passing through each stand in a given timeshould be the same. This has already been described in U.S. Patent3,334,502, of Heindel et al. which issued Aug. `8, 1967. If the screwsetting of the rolls of any one stand or the speed of the motor on thatone stand is altered so as to alter the mass ow rate through that stand,then it is desired that the other stands should be capable of adjustingto this disturbance in reference to the screwdown setting and/ or thespeed setting of those other stands. In wide strip rolling, such as herecontemplated, the volume of the metal is practically unaffected by theamount of work it receives and the lateral spread of the strip is sosmall that its width may be taken as substantially constant. It has beenknown in the prior art to establish a desired speed cone relationship ofthe various stands of a tandem mill such that a desired operating speedrelationship is established between each stand and the other stands ofthe mill to determine a desired distribution of the total reduction loadof the respective stand motors for the desired thickness reduction byeach stand to be achieved by passing the strip through the tandem mill.It has been the practice for mill operators to establish a desired speedof operation for each stand at a thread speed operation of the mill, andthis same generally desired speed relationship of the respective standsis held for acceleration up to a run speed of operation of the tandemmill. Further it has been the practice in the past to provide aninterstand tension sensing device or looper which is-operativethroughout the entire speed range to adjust one of the respectivesucceeding stand speed regulator or preceding stand speed regulator forthe purpose of maintaining a substantially constant interstand tensionbetween each of the respective pairs of stands. Further, it has beenknown in the prior art, as shown by U.S. Patent 1,145,880 of Clark, tosense the tension between "ice a given pair of stands and to adjust thescrewdown setting of either the preceding stand or the succeeding standrelative to the sensed interstand tension throughout the entire millspeed range for the purpose of maintaining a substantially constanttension between that pair of stands.

IIn the prior art control of a tandem rolling mill the above twoapproaches have been generally used for the control of interstandtension for the purpose of regulating the interstand tension to remainsubstantially constant. The second approach of tension regulation,feeding the sensed tension error signal to the screwdown speed regulatorof one of the adjacent stands, can have the advantage of in someinstances being faster, it can be made more stable and the variations inthe delivery gauge of the final product can be reduced due to theinherent control of mass flow provided by the screwdown adjustmentapproach.

However, a difficulty can arise when the interstand tension is regulatedto remain substantially constant by adjustment of the screwdowns whenthe rolling mill is running at low speeds such las thread speed orbelow. If the run speed of a rolling mill is in the order of 5000 feetper minute, the typical thread speed may be in the order of 10% of runspeed or about 500 feet per minute. At low mill speeds, such as threadspeed or below, the required screwdown adjustment may involve anappreciable movement by the screws in order to have any control effectover the interstand tension. At the extreme situation where the mill isactually stopped, the screws can run any distance and no change ininterstand tension will occur.

If, when the operator is threading a mill, he has not properly set themill speed cone, or the speed relationship :between the respectivestands, as necessary for stable operation of the mill, and the variousscrew positions are not at the required relationships with each other,the resulting interstand tensions will not be as desired and will notremain substantially constant. When the tension regulators operativewith the screwdowns are then activated, these will cause the respectivestand screws to run up or down as needed in an etort to reduce thesensed tension errors to zero. However, since the mill is now operatingat low speed, it can be necessary for the screws to run a large distancebefore the tension error between a given pair of stands is reduced tozero. This can cause a severe disturbance to the mill set up, and if theinitial tension error is large enough, the screws can either losecontact with the strip by opening too much or run-down to the extentthat the strip is broken. The latter fact poses a serious limitation onthe tension control by the screws and the control arrangement may haveto be automaticaly turned oft when the mill is operated below a certainminimum speed such as thread speed. However, it is desired, and anobject of the present invention, to control the interstand tensions assoon as the strip is entered into the respective stands of the mill fromzero. speed of operation to top mill speed.

SUMMARY OF THE PRESENT INVENTION The interstand tension control heredisclosed employs a combination of screwdown and stand speed` changes toobtain the desired substantially constant interstand tension regulationthroughout the entire mill speed range. This is accomplished byproviding an interstand tension sensing device, such as a tensiometer,between each pair of adjacent stands of the rolling mill and providingan actual interstand tension signal which is compared with a desired orreference interstand tension signal to generate an interstand tensionerror signal for each pair of stands. For low speed operation of themill, this tension error signal relative to each pair of lstands issupplied to the coarse speed regulator of one of that pair of adjacentstands, such as the preceding stand, to provide in plied to the coarsespeed regulator of one ot that pair of adjacent stands, such asthepreceding stand, to provide in this manner a proper speed cone speedsetting for that stand to reduce any interstand tension error and tomaintain a substantially constant and desired interstand tensionrelationship between that pair of stands, regardless of the actual lowoperating speed of the mill at this time. When the operating speed ofthe mill is increased above this predetermined low speed operation,which in fact may be the thread speed operation of the mill or in theorder of of the established run speed of the mill, this interstandAtension error signal is removed from the coarse speedy stand regulatorand is applied to this screwdown speed regulator or controller for thesucceeding stand of the particular pair of stands being regulated toreduce the interstand tension error. After the strip has passed throughall of thel stands of the rolling mill, with each stand or pair ofstands being controlled to be operative as previously described, and thestrip has taken a sufficient number of wrapsV around the windup reel,the operator can now change the operating speed of the rolling mill tobe increased torun speed. The speed cone of the rolling mill will atthis time be established such that the operating speed of the rollingmill can increase to run speed without being subject to the undesiredadjustment of the respective screws as previously described, and themill should be subject to no objectionable disturbance in the nowprovided mill speed setup relationships, and the interstand tensions cannow be maintained substantially constant as desired.

BRIEF DESCRIPTION OF THE DRAWINGS FIGURE l is a general illustration ofthe interstand tension control in accordance with the present inventionapplied, in the detailed example, to regulate the interstand tensionbetween the second and third stands of a tandem rolling mill;

FIG. 2 illustrates in greater detail one suitable circuit arrangementprovided for adjusting the interstand tension of the strip between thesecond and third stands of a tandem rolling mill;

FIG. 3 illustrates in greater detail the operation of the deadband gateshown in FIG. 2.

In the normal operation of a tandem rolling mill, a thread speed bus canbe energized to regulate the operating speeds of the individual standdrive motors in accordance with individually setreference speeds toprovide a desired thread speed cone relationship, and this is done priorto the steel strip entering the first stand. With the entire rollingmill operating at thread speed, the steel strip now enters the firststand and passes through the succeeding stands until it is wrapped ontothe tension reel. The thread speed is generally in the order of 10% ofrun speed, or for a mill having a runspeed of 5000 feet per minute inrelation to the delivery speed of the strip from the last stand andentering the windup reel; the thread speed at this point would be in theorder of 500 feet per minute. The low mill speed of operation hereconsidered is up to thread speed, and can be anywhere from zero speed toabout 10% of the mill run speed, .and it generally applies to theinitial portion of the total length of the strip that passes through themill. It is desired in this regard to increase the total on gauge stripdelivered from the mill even during this initial low speed of operationwhen the mill is being threaded and before the mill is energized by themaster run bus to increase its operating speed up to the desired runspeed of operation.

v Tension control between stands by operation of the adjacent screwdowncontrollers has several advantages. However, if the operator has notestablished a good speed c one relationship for the operation of therespective stands of the mill, and has an incorrect setup in thisregard, the prior art automatic tension adjustment control by operationof the screwdowns would require an undesired amount of screwdownadjustment and for this reason it is here desired that the tensioncontrol by screwdown adjustment be deenergized until the operator hasraised the operating speed of the mill up to at least thread speed ofoperation. The teachings of the present invention enable theestablishment of a good speed cone setup during the low speed andthreadingV operation of the mill, such that when itis desired toincrease the operating speed of the mill up to run speed this can bedone without requiring substantial change in the speed cone setup. Itshould be understood that it is sometimes desirable to thread the millat a speed below 10% of run speed for various reasons; and the 10% ofrun speed should be understood to be a maximum thread speed, and manytimes depending upon the characteristics of the strip supplied to themill a lower threading speed may be desired. It should be furtherunderstood that the mill setup for a given strip usually is changed forsucceeding strips, if the succeeding strips do not have the samethickness and material characteristics as the previously run strip whichpasses through the mill. In accordance with the present invention, thetension error signal is utilized below maximum thread speed foradjustment of the stand speed reference to initially establish the speedcone relationship as the strip passes through the succeeding pairs ofstands of the rolling mill; and after this desired speed conerelationship is established and the strip has taken a few wraps on thewindup reel and the mill is increased in speed above the maximum threadspeed of operation, the stand speed regulators are no longer adjusted bythe interstand tension error signal and only the screwdown controller isadjusted by this signal. At this point of operation, the control of thepresent invention has established the right speed cone relationship forthe respective stands of the mill and the acceleration of the mill up torun -speed provides no problems in terms of excessive screwdownadjust-ment, and the position of the 'screwdowns at this point is properas well as the speed regulation of the individual stand speedregulators. It should be understood that a proper speed conerelationship at 100 or 200 feet per -minute is substantially the same asthe speed cone relationship for the run speed of 5000 feet per minute.In particularly cold mill operation, succeeding strips changesubstantially from earlier run strips in terms of desired gauge and withof -strip and material characteristics and the like, and :for thisreason the speed cone relationship for a particular strip is usually notthe same as the desired speed cone relationship for succeeding strips.Therefore the speed cone relationship =for a tandem cold mill requirescontinual adjustment for the various strips which pass through the mill.

DESCRIPTION OF THE PREFERRED EMBODIMENT is supplied from a potentiometer18 to a second input of the tension regulator 16. The latter tensionregulator 16 is operative to provide a tension error signal by adifference comparison of the actual interstand tension signal receivedfrom the device 14 and the desired interstand tension signal receivedfrom the potentiometer 18, which error signal it applies through aninitially closed contactor 20 to the stand speed regulator 22 providedfor the coarse speed regulation of the drive motor 24 for the previousstand 10. The contact 20 is closed below maximum thread speed, which isin the order of 10% of the run speed of about 5000l feet per minute forthe rolling mill shown in FIG. 1. A second contact 26 is normally openbelow the maximum thread speed.

As the strip initially moves from the rst stand to the second stand 10the tension sensing device 30 is operative with a generally similarinterstand tension regulation control 29 to control the drive motor forthe first stand for mill operation below maximum thread speed andoperative with the screw-down controller 31 for the second stand abovemaximum thread speed. When the strip passes from the second stand intothe third stand 12, the tension sensing device 14 becomes operative tosense the interstand tension between the second and third stands. As thestrip moves from the third stand into the fourth stand, the tensionsensing device 32 is operative with a similar tension regulation control33 to control the interstand tension between the third and fourth stand.As the strip moves from the fourth stand into the ifth stand, a tensionsensing device 34 is operative with a similar control to control theinterstand tension between the fourth and fifth stands. As the stripmoves from the lifth stand onto the windup reel 36, and takes a fewwraps around the windup reel as desired by the mill operator, theoperator can now press a run speed button (not shown) which causes thedrive motors of the respective stands to be energized in a well knownmanner by a run bus as compared to a thread bus, such that the operationof the mill accelerates from the thread speed to the run speed. Thespeed cone relationship established as above described during thethreading operation is retained in general during the acceleration andwhile the mill is operating at run speed.

When the operator pushes the run button to cause the rolling mill toaccelerate from thread speed up to run speed, the contact shown inFIGURE 1 opens and the contact 26 is closed such that any interstandtension error signal supplied yby the tension regulator 16 will now beno longer supplied to the stand speed regulator 22 and instead will beapplied to the screwdown controller 38 for the third stand 12 such thatscrew adjustments are made as required to maintain a substantiallyconstant tension between the second and third stands of the rollingmill. In regard to the output signal from the tension regulator 16 it isthe speed regulation of stand 2 relative to stand 1 and stand 3 which isunchanged by the tension regulator 16 above the maximum thread speed ofthe mill. This means that the ratio of v1 to v2 and the ratio of v2 tov3 is not changed by the tension regulator 16 above the maximum threadspeed operation of the rolling mill. A similar tension regulator controlis provided in accordance with the teachings of the present inventionbetween stand 1 and 2 and between stand 3 and 4 and between stand 4 and5, as is illustrated in FIG. 1. Each of these tension regulator controlsoperates in succession as the strip passes between the respective standsto provide the desired speed cone relationship for the entire mill priorto the operator pressing the run button and accelerating the mill frommaximum thread speed or below up to desired run speed.

In FIG. 2 there is shown in greater detail the interstand tensioncontrol arrangement of the present invention including a master speedreference potentiometer operative through an operational amplifier 52for energizing a bus 53 to control the pattern operating speed of allstands of the rolling mill. A potentiometer 54 is operative with thefirst stand for providing a coarse speed adjustment function, apotentiometer 56 is operative with the second stand for providing acoarse speed adjustment of the operating speed of the second stand, apotentiometer 58 is operative to provide a coarse speed regulation ofthe third stand, a potentiometer 60 is operative to provide a coarsespeed regulation of the fourth stand, and a potentiometer 62 isoperative to provide a coarse speed regulation of the fifth stand. Itshould be understood that the potentiometers 54, 56, 58, 60 and 62 arethe coarse speed motor operated rheostats for the respective stands ofthe mill. The signal from the actual tension sensing device 14 and thesignal from the desired tension reference potentiometer 18 are summedtogether at a summing junction 70 operative with an operationalamplifier 72 for providing an amplified tension error signal at outputjunction 74 which passes through a dead-band gate device 76, which willbe described in greater detail in relation to FIG. 3. A contact 78 isprovided for manual operation of the rolling mill in that for thetension regulation control shown in FIGURE 2 to be properly operativethe operator should observe that strip should be present between thesecond and third stands of the mill; otherwise the tension sensingdevice 14 will provide an erroneous signal.

When strip is present between the second and third stands, as forexample can be sensed by a roll force sensing load cell device 80 asshown in FIGURE 1 operative with the third stand of the rolling mill,the contact 79 can be closed through switch 83 when the roll force loadcell device 80- indicates that strip is present in the third stand ofthe rolling mill. If it is desired to make certain that the tail end ofa given strip has not left the second stand and is present in` the thirdstand, the output signal from a roll force load' cell device 82operative with the second stand can -be used in combination with theoutput signal from the roll force load cell device 80 of the thirdstand, for example in a well known AND gate 81, to provide an outputsignal through switch 85 only when strip is present between the standstwo and three for the purpose of closing the contact 78 when thiscondition occurs.

The contact 20 is normally closed, for low speed operation of therolling mill, to apply any tension error signal passing through thedeadband gate 76 to an operational amplifier to energize the motor 92for adjusting the position of the rheostat 5'6 which in turn isoperative with the second stand speed controller for the purpose ofadjusting the operating speed of the second stand in a direction toremove any tension error signal supplied at the output terminal 74 ofthe operational amplifier 72.

The function of the`deadband gate 76 is to prevent tension control inthis manner by a small error signal less than about 2 or 3% of thetension reference value established by the potentiometer 18. An errorsignal above 10% of the desired interstand tension value will passthrough the deadband gate and be operative with the motor 92 to adjustthe operating speed of the stand two. As long as the error is greaterthan 2 or 3%, the error signal is applied to the motor 92 to bring thetension error down to zero, and the error signal is continuously applieduntil the tension error drops to about 2 or 3% before the deadband gatewill no longer pass the error signal to the amplifier 90. The deadbandgate 76 is operative in a similar manner relative to an opposite tensionerror signal.

It should be understood in general that each of the motor operatorrheostats 54, 56, 58, 60 and 62, operative with the respective stands ofthe rolling mill, are initially preset manually to provide a desiredspeed cone relationship. After this initial manual setting the tensionregulating control of the present invention is operative to adjust thesesettings and change the operating speed of each particular stand drivemotor in relation to the other stand motors, to thereby realign thespeed relationship or speed cone settings of the respective motors asrequired to provide a substantially constant and desired interstandtension relationship between the respective adjacent stands of therolling mill. Therefore, when the operator presses the run button tobring the operating speed of the rolling mill up to run speed, the bus53 common to each of the stand speed rheostats has an increased voltageapplied to it to thereby increase the operating speeds of the respectivedrive motors; however, the established relationship between those speedsis held constant as the voltage of the bus 53 increases to run speedvalue. The tension control arrangement shown in F IG. 2 is thereforeseen to be operative to adjust the speed setting of 'the potentiometer56 operative with the drive motor for the second stand in relation tothe speed settings of the potentiometers for the other drive motors.Further, it should be understood that an interstand tension controlarrangement similar to that shown in FIG. 2 is provided for each of theinterstand tension sensing devices 30, 14, 32 and 34 as shown in FIGURE.l, and is operative with the respective stand potentiometers 54, 56, y58and 60. After the strip enters the tension reel 36, and has taken a fewwraps to the satisfaction of the operator, the operator presses the runbutton and opens the contact 20 and closes the contact 26 for each standtension control such that any existing tension error signals relative tothe strip present between the respective stands is now supplied to thescrewdown speed regulator operative with the screwdown controller of thenext adjacent stand. The contact 20 for each pair of stands is open suchthat the tension error signal supplied at the output of the tensionerror operational amplier is no longer operative to adjust the speedsetting of the drive motor of the associated stand. Thusly, since thissame operation holds true for the interstand tension controlarrangements for each of the pairs of stands, the now established speedcone relationship for the entire mill is no longer changed by tensionchanges occurring between the respective stands of the mill. Anyadjustment of the respective interstand tensions after the operator haspressed the run button `is provided by proper adjustment of thescrewdown controllers of the succeeding stand relative to the particularinterstand tension which needs correction. f,

The deadband gate 76 is operative such that small errors do not resultin changes of the settings of the respective stand speed rheostats sincenoise and other objectionable signals otherwise would result inundesired changes in the speed cone relationship and unstable operatingconditions might result for the interstand tension control arrangement.The deadband gate 76 is operative such that when the tension errorsignal is above the desired tension reference value by somepredetermined amount, such as by an amount equivalent to about 10%, thefull error signal is applied to the stand motor speed regulator. As theerror signal is corrected and therefore decreases until it reaches avalue in the order of 2%, the error signal is no longer applied to thespeed regulator. Should the error then reverse itself, nothing happensuntil it reaches this predetermined amount, such as a value in the orderof 10%, when it again passes through the deadband gate for its fullvalue in the opposite direction until the tension error signal iscorrected to a value where it is within 2% of the desired interstandtension at which time the tension error signal is again removed. Thisavoids unstable operation of the interstand tension regulationoperation.

The contact 78 is used to manually load up the mill until some tensionis present between the respective stands to be sensed by the tensionsensing devices. Upon closing the contact 78 this causes the speedadjustment of the second stand motor to be such as to provide thedesired interstand tension between the second and third stands. As thestrip passes through the entire mill, similar con trols successivelyoperate for the respective interstand tensions between the iirst andsecond stand and then the second and third stand and then the third andthe fourth stand and then the fourth and tifth stand such that, when thestrip has entered the tension reel 36 and been Wrapped around a coupletimes, the desired speed cone relationship is established for the mill.After this the respective lscrewdown speed regulators can be energizedby any remaining tension error signals. If desired, the roll forcesignal from the second stand can determine the energization of theinterstand tension control arrangement for the strip between the irstand second stands, and the roll force signal from the third stand toindicate that the strip has entered the third stand can be operative toenergize and make operative the interstand tension control arrangementfor the strip between the second and third stands, and so forth, as thestrip passes through the mill and enters the fourth stand and then thefifth stand in succession. As an alternate arrangement if desired atachometer operative with the drive motor of the fifth stand can sensewhen the rolling mill is operative at maximum threadspeed or above andcan be operative to deenergize the interstand tension regulatingapparatus operative with the speed regulators and instead send therespective interstand tension error signals to the successive standscrewdown controllers, by energizing a relay 25 for each of theinterstand regulators, operative to open the contact 20 and close thecontact 26 and corresponding contacts for the other interstand tensionregulators. A well known bistable amplier can be utilized for thispurpose and which is not operative to switch its state of operationuntil a predetermined positive value input signal is applied to itsinput; it could be adjusted to indicate that the maximum thread speed ofoperation has been reached and the mill is operating at this level orabove.

In FIG. 3 there is illustrated in greater detail the operation of thedeadband gate 76. It should be understood that the embodimentillustrated in FIG. 3 is not intended to limit the scope of the presentinvention but rather to provide one suitable illustration of anoperative form of deadband signal gate device. In general, a deadbandgate per se is well known to persons skilled in this art and there areseveral suitable devices available on the open market at the presenttime which would be suitable for this purpose. In FIG. 3 the elementscorresponding to those shown in FIGURE l and FIGURE 2 have been givenlike reference numerals. The stand speed rheostats 54, 56, 58 and 60 areadjusted by the interstand tension regulating arrangement of the presentinvention. In addition there are line speed manually adjusted rheostatsprovided for the operator to preset the speed cone of the mill beforemetal is in the stands, with the usual practice that once the operatorsets the ne speed rheostats he leaves them along and relies upon theoperation of the tension control arrangement to adjust the speedrheostat as shown in the present invention.

In FIG. 3 there is shown the dead zone gate 76 to include four bistableamplifier circuits 100, 102, 104 and 106. These are respectivelyoperative with the relay windings 107, 109, 111 and 113, for determiningthe operationv of the associated contacts 108, 110, 112 and 114. It isassured that a negative tension error is initially present from thestart, and is present until strip is entered into the mill. When thetensiorneter device senses an interstand tension condition it applies anactual tension signal to input 15 of amplifier 72. The tension referencesignal provided by the potentiometer 18 is applied to input 19. Theerror signal from the output of the ampliiier 72 is applied to one inputof each bistable amplifier through conductor 21. When this error isgreater than about 2%in a positive direction it will cause the bistableamplifier 104 to energize the Winding 111 to close contact 112. As theerror signal becomes greater than about 10% in a positive direction, itwill cause the bistable amplier to energize the winding 107 to closecontact 108. The latter contact 108, when closed, energizes the relaywinding 117 to close contacts 116 and 119. The contact 116 seals thewinding 117 on itself through the already closed contact 112. Thus,contact 108 closes at an error of +10% and this applies the tensionerror signal from the amplifier 72 to the motor 92, until the tensionerror is reduced below -i-2%, when the contact 112 opens to remove theenergization from winding 117 to open contact 119. It should beunderstood that the contact 108 opened prior to the tension error beingreduced to -l-2%. For a negative tension error, the bistable amplier 106closes contact 114 at a tension error of about 2% and the bistableamplier 102 closes contact at a tension error of about -l0%, such thatthe motor 92 is energized to increase the tension when the tension errorbecomes -l0% and is no longer energized when the tension error reaches2%. The bistable amplifiers including the associated relay per se arevery well known devices and are available at the present time 9 in theopen market as Westinghouse Electric Corporation catalog item P-801.

In this manner the speed setting for each stand is adjusted by a motorcorresponding to the motor 92 shown for stand two, such that theinterstand tension condition fo reach pair of stands is in accordancewith a'desired tension value as the strip passes through the respectivestands ofthe mill.

In the normal operation of the interstand tension control arrangementshown in FIGURES 1, 2 and 3 the interstand tension condition between therespective stands is succeedingly corrected as the strip passes throughthe mill from the rst stand to the windup reel such that if an indicatorlight were operative with the output of the bistable amplifier of eachtension control arrangement, such as the bistable amplifier 100, thiswould indicate when the tension condition between the respective standswas such that it was less than positive 10% in error relative to thedesired interstand tension condition; a similar light operative with theoutput of the bistable amplifier of each tension control arrangement,such as the bistable amplifier 102, would indicate that the actualtension condition was no greater than 5% negative 10% away from thedesired interstand tension reference condition. These lights could belocated at an operators console and indicate to the operator that thedesired rough correction regulation had occurred between the respectivestands, and that it was alright to press the run button to cause themill to now increase its operating speed `from maximum thread speed orbelow up to run speed. Since it is highly desirable to control therespective interstand tensions as soon as the strip is threadedregardless of the operating speed of the mill, from zero to top runspeed, the control arrangement in accordance with the present inventionuses a combination of screwdown adjustment and stands speed changes toobtain the desired tension regulation between the stands throughout theentire mill speed range. In the above description reference was made tothe interstand tension control between the second and third stands ofthe five stand cold mill, but it should be understood that a similarinterstand tension control arrangement can be applied to each of theother interstand tension conditions in a tandem mill with any number ofstands. As soon as the head end of the strip reaches each succeedingstand and some tension is detected by the tension sensing deviceoperative ahead of that stand the tension regulator control operativewith that device is turned on. This can also be accomplished by a rollforce sensing load cell device operative with each succeeding stand toindicate that strip has now entered a given stand and is thereforepresent up to that stand. With the mill operating at low speed, thetension between any pair of stands will be controlled by changing thespeed of the preceding stand. In order to obtain a smooth operation whenthe speed of any stand is increased, above thread speed, the tensionerror is not fed directly to the speed regulator of that stand but isfed instead to the speed controlling motor operated rheostat of thatstand. The tension error or difference between the desired tension andthe actual interstand tension is sensed by the tension sensing deviceand is fed through a deadband gate to the motor operated rheostat forthe number 2 stand. As illustrated the deadband has a hysteresis effectwhich means that the gate is closed to pass the error signal when theerror is greater than a certain percentage of the desired tensioncondition and remains closed until the error is reduced to apredetermined lower value of the tension error condition. This gate canbe made to operate in a percent basis by using the tension referencesignal as a bias to the provided tension error detection bistableamplifiers.

While the mill is being threaded, the tension regulator controls willchange the position of the motor operated rheostats controlling thespeed of each stand to obtain the desired interstand tensionrelationships as called for by the tension reference setting rheostatsfor the respective pairs of stands. When the head end of the stripreaches the tension or windup reel, all the stands speeds should be atthe appropriate values for the desired interstand tensions for the nowexisting screw positions and the tension errors should be reducedbetween the stands to substantially zero. In other words, the speed coneof the mill will be correct for the existing screwdown settings. Sincethe speed correction for each stand has been made through the motoroperated rheostat, when the mill is accelerated each stand willmaintatin the correct relationship of speed to the other stands. Thiswould not be true if the tension error were directly fed to the standspeed regulators. When the threading of this mill is completed, thetension error signal is removed from the motor operated rheostat drivemotor and applied to the appropriate screwdown controller for the nextsucceeding stand through the stand deadband gate. The tension will nowbe controlled by operating on the screwdown mechanisms with all theadvantages provided by the previous speed regulation. However, since therelative speeds of the stand are now correct, and the initial tensionerrors are substantially zero, no excessive running of the screwdownmechanisms will result.

It should be understood that it is within the scope of the presentinvention to control interstand tension for maintaining constant mass owconditions between a given pair of stands by adjusting the speed andscrewdown of either the previous or the succeeding stand. For example,if the interstand tension is too high, the product of thickness hp andstrip velocity vp of the previious stand, should be made equal to theproduct of thickness hs and strip velocity vs of the succeeding stand;this can be done by raising the screwdown setting of the previous standto increase hp or by increasing the previous stand speed vp toeffectively reduce the tension between the stands; this can also be doneby lowering the screwdown setting of the succeeding stand to reduce hsor by decreasing the succeeding stand speed vs to effectively reducethis same interstand tension. Further, it is within the scope of thepresent invention to provide an overlap in the operation of thedescribed speed adjustment and the screwdown adjustment, for example, bycontrolling the opening of contact 20 for any given interstand tensioncontroller to occur at a different and perhaps greater mill speed thandoes the closing of the contact 26 such that for a selected range ofmill speed operation overlapping the thread speed, the speed adjustmentand screwdown adjustment can be applied together. Also, it is within thescope of this invention to narrow the effective deadband provided by thedeadband gate 76 as compared to the specific example set forth in thisspecification. As generally shown in FIGURE l, the teachings of thepresent invention can be utilized in conjunction with the well knownX-ray gage coarse thickness control system applied to stand one and theX-ray gage Vernier thickness control system applied to stand five of therolling mill.

While the present invention has been described with a certain degree ofparticularity it should be understood that various modifications andchanges thereof can be made within the scope and spirit of the presentinvention.

I claim as my invention:

1. In control apparatus for a strip rolling mill having at least twosuccessive stands of spaced roll members, the combination of striptension sensing means responsive to the actual tension of said striprelative to the passage of said strip between said two stands,

driving means' for the roll members of at least one of said stands toprovide a desired operating speed for the latter said roll members,

spacing control means operative with the roll members of at least one ofsaid stands to provide a desired spacing between the latter said rollmembers,

` and thickness control means operative with said driving means and withsaid spacing control means for controlling the thickness of said stripin accordance with the difference between said actual tension and adesired strip tension between said stands, with said thickness controlmeans being responsive to the speed of said strip passing between saidstands to selectively operate in a predetermined manner with saiddriving means and with said spacing control means in accordance withsaid strip speed.

stands in accordance lwith said tension error when said strip speed isabove said predetermined value for reducing said tension error.

5. The method of controlling the delivery thickness of strip material ofclaim 4, including,

controlling the operating speed of the rst of said pair of stands whenthe strip speed is below the predetermined threadspeed of said mill, and

controlling the spacing between the roll members of the second of saidpair of stands when the strip speed 2. The control apparatus of claim 1,including lo is above said thread speed of said mill.

said thickness control means being responsive to the 6. The method ofclaim 4, including speed of said strip passing between said stands beingmonitoring the actual tension of the strip material beone of below apredetermined strip speed or being tween each succeeding pair of saidstands to deterabove said predetermined strip speed and being op- 15mine the tension error between each succeeding pair erative with saiddriving means below said predeof stands, termined strip speed and beingoperative with said controlling the operating speed of the iirst standof each spacing control means above said predetermined said pair ofstands in accordance with the tension erstrip speed. ror between thatpair of stands to reduce the said ten- 3. The control apparatus of claim1, sion error,

with said driving means being operative with the iirst and controllingthe spacing between the roll members of said two stands and with saidspacing control of the second stand of each pair of stands inaccordmeans being operative with the second of said two ance with thetension error between that pair of stands, stands to reduce said tensionerror.

said thickness control maens being operative with 7. The method ofcontrolling the thickness of strip masaid first stand driving meansbelow a predetermined strip speed to control the operating speed of theroll members of said first stand to reduce said tension difference.

and said thickness control means being operative with said spacingcontrol means of the second stand above said vpredetermined strip speedto control the spacing between the roll members of the second stand toreduce said tension difference.

4. The method of controlling the delivery thickness of terial passingthrough the plural stands of a tandem rolling mill, includingdetermining the actual tension of said strip material between a pair ofsaid stands,

determining the speed of said strip material,

controlling the speed of at least one of said stands for a irst range ofsaid strip speed in accordance with said actual tension for controllingsaid actual tension,

controlling the screwdown setting of at least one of said strip materialpassing through the plural stands of a tandem rolling mill, with each ofsaid stands including spaced roll members, said method includingmonitoring the actual tension of said strip material passf ing between apair of said stands to determine the tension error in accordance withthe diterence between a desired tension condition and the actualtenstands for a second range of said strip speed in accordance with saidactual tension for controlling said actual tension.

References Cited UNITED STATES PATENTS sion condition of said stripmaterial, d-e-e-al "-7-27221 monitoring the speed of said strip materialpassing be- 355:9 18 12,1967 Wallace 72-12 X tween said pair of stands,

controlling the operating speed of one of said pair of stands inaccordance with said tension error when said strip speed is below apredetermined value for reducing said tension error and controlling thespacing between the roll members of one of said pair of MILTON S. MEHR,Primary Examiner Us. c1. Xn. soA 72g-12, is

